Nowadays, wireless telecommunication is at least as common as the more conventional wired form of telecommunication. An increasing part of the population in almost every country uses wireless radio telephones, especially cellular telephones, in the daily life. The term "radio telephone" includes all portable equipment, which may be used for radio communication, e.g. cellular telephones, so-called personal communicators or organizers, etc. The predictions for the future further indicate an even more frequent use of such telephones, and in a not too distant future it is possible that wireless telecommunication systems will become more common than the ones using wires.
In mobile telephone systems, or so called cellular telephone systems, a telephone company--public or commercial--provides a number of geographically distributed base stations. Each base station is arranged to cover a certain area, a so-called cell, and the various areas are selected in such a way, that they to the highest possible extent together cover a continuous geographical area. The base stations are arranged to forward incoming and outgoing calls to and from any mobile stations, such as radio or mobile telephones, present in the area, and hence each base station is connected to the public wired telecommunication network. A mobile station in a modern telecommunication system, such as GSM (Global System for Mobile Service), has two main modes of operation, the first of which being during ongoing calls with an intense communication between the mobile station and the base station in question. The second mode of operation is the stand-by mode, i.e. when the mobile station is not serving any ongoing call, a certain communication with the base station, however, still occuring at a regular basis. For instance, when the user turns on his mobile station, the mobile station will search for available base stations and select the one with the highest signal strength. Then the mobile station reports its presence to the selected base station. The mobile station will then regularly receive information from the base station, for instance frequency synchronization information. If the signal strength turns out to be lower than the signal strength of any other available base station, a switch to the latter normally occurs (so-called hand-over). Furthermore, a bidirectional so-called handshake communication takes place between the mobile station and the base station, when an incoming call is to be established. Thanks to the above it is possible to provide services like roaming (the system detects and keeps record of subscribers for the moment being abroad), SMS (Short Messages Services; transmission of short text messages to be presented on the telephone display), and CB (Cell Broadcast; broadcast messages from, e.g. a public authority to several mobile telephones present in a cell).
Other kinds of wireless telecommunication are known, which are not connected to the public wired telephone network. One example is dispatching applications, i.e. non-public radio systems used by e.g. taxi services, the police authorities, or transportation companies. Another example is local or global paging systems, and yet another one is cordless telephone systems, such as DECT (Digital European Cordless Telephone System).
For speech or data information to be transmitted in a wireless way in a communication system mentioned above, transmitters as well as receivers of electro-magnetic energy are required. In the transmitter as well as the receiver the information is electrically processed, regardless whether it is an analog or digital communication system. The transmitter as well as the receiver are connected to conversion means, such as an antenna, for conversion between electrical and electromagnetic energy. The wireless information transmission is effected by the radio frequent electro-magnetic fields generated around the transmitter antenna and propagated away from the antenna, then by the speed of light reaching the receiver antenna.
Various demands are placed upon the mobile station antennas in a modern mobile telephone system, since the antennas are heavily used during ongoing calls as well as during stand-by mode, as described above. Financial demands as well as claims for cost-efficiency are obvious. Another common demand is a long operation time between battery chargings, and to be able to fulfil such a demand the mobile station has to preserve the amount of electrical energy available, thereby requiring a high antenna conversion efficiency. Usually, the antenna and signal quality requirements are stricter during ongoing calls than during stand-by mode.
Furthermore, the physical dimensions of the mobile telephone are very important. For a mobile telephone to be regarded as useful by the users of today, a low-weight, compact and yet robust format is required; preferably, the mobile telephone should fit inside jacket pockets, handbags, etc, or it should be possible to carry the telephone by the waistbelt by means of a beltclip. The demands on compactness and robustness obviously also apply to the mobile telephone antennas.
During ongoing calls a mobile telephone is normally positioned in close vicinity of the user, and, as a consequence thereof, the user's body--the hand and the head in particular--will inevitably affect the signal properties of the antenna. Such interaction between the body and the near field of the antenna usually results in deteriorated antenna properties, mainly due to reflections and absorption losses. Specifically, this signal degradation will increase, if the distance is reduced between the antenna and the body, and obviously, there is a severe difficulty in achieving a high antenna efficiency as well as physical compactness and robustness at the same time.
Various antenna types are used in the mobile stations of today, for instance half or quarterwave whip antennas, dipole antennas, helix antennas, etc. Different antenna types are suitable to different extents when it comes to signal properties and antenna efficiency. For instance, the helix antenna is often used, since--thanks to its helical shape--it may be given a compact size and a good resistance against damages. However, it does not show an antenna efficiency as good as that of a half-wave whip antenna.
Due to space considerations mobile telephones are furthermore often provided with extendable whip antennas, which in some cases are supplemented by separate external antennas for use in stand-by mode, since a whip antenna fully retracted into a metal telephone housing is isolated from its surroundings with respect to its signal emission. The international patent publication WO 93/01659 from Motorola, Inc., discloses a radio telephone, whose antenna may be moved between a retracted and an extended position. The telephone has two different operating modes, in dependency of the antenna being extended or not. These operating modes may for instance be related to the display being lit or not, the keypad being engaged or disengaged, or to different levels of transmission effect.